mlxsw: spectrum_switchdev: Prepare for VxLAN FDB notifications

VxLAN FDB notifications need to be handled differently than bridge FDB
notifications, so initialize the work item based on the received
notification and rename the invoked function accordingly.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>